1. Field of the Invention
The present invention relates to a display device and a driving method of the same.
2. Description of the Related Art
A conventional display device includes a plurality of pixels arranged in a matrix form, and an image is displayed by controlling light strength of each pixel according to given luminance information. An organic light emitting display is a display device which displays an image by electrically exciting a fluorescent organic material and allowing the fluorescent organic material to emit light. The organic light emitting display is a self-emitting display device, and includes low power consumption, a wide viewing angle, and a fast pixel response speed, thereby easily displaying a high quality motion picture.
The organic light emitting display includes an organic light emitting diode (“OLED”) and a thin film transistor (“TFT”) which drives the OLED. The TFT is classified as a polysilicon TFT and an amorphous silicon TFT according to the type of active layer.
When an active layer of the TFT is formed, a deviation is generated in a threshold voltage of TFTs in one panel due to a non-uniform process. Further, as the TFT continuously supplies a current to the OLED, the threshold voltage of the TFT may be changed. When a deviation is generated in the threshold voltage of the TFTs or when the threshold voltage is changed, the TFTs flow a different current in response to an identical data voltage, whereby brightness uniformity of a screen is deteriorated.
In order to remove the influence due to a deviation in a threshold voltage of the TFT, a conventional method exists which allows a driving current which flows by the TFT not dependent on the threshold voltage by storing the threshold voltage in a capacitor. The conventional method includes storing a threshold voltage in a capacitor according to a scanning signal of a previous scanning line before storing a data voltage in a capacitor according to a scanning signal of a current scanning line. Thus, a capacitor is charged or discharged until a voltage between a gate and a source becomes a threshold voltage by flowing a current to the TFT.
In the conventional method, a compensation time in which a threshold voltage is stored in the capacitor corresponds to a period of the scanning signal of the previous scanning line and is equal to or less than one horizontal period. However, if a voltage between a gate and a source approaches a threshold voltage, because a current flowing to the TFT rapidly decreases, a charge or discharge speed of the capacitor becomes slow. Accordingly, since the capacitor cannot be fully charged or discharged in a compensation time period, a threshold voltage may not be stored in the capacitor and thus, brightness uniformity of a screen due to a deviation of a threshold voltage may still be deteriorated.